The ATSC television standard incorporates 5.1 channel sound. In the past TV producers focused only on high-definition video, but as that has become more common, they are increasingly upgrading network broadcast programming from stereo to 5.1 audio.
A significant portion of primetime high-def broadcast programming is now being aired with multi-channel sound. It will only increase in the future. It's not surprising, since audio has frequently been remixed to 5.1 for the DVD release of a TV series; it only makes sense therefore to do it for the original broadcast.
If you want to hear the multi-channel sound, you will in almost all cases need to buy a separate multi-channel amp and at least some separate speakers. You may be able to use the TV's built-in speakers for the main left and right channels, depending on what quality system you're looking for.
While broadcast multi-channel audio is now 5.1 (left and right main channels, center channel, left and right surrounds, and the .1 subwoofer channel, sometimes called LFE--low frequency effects), the latest audio receivers are more commonly equipped for 6.1 or 7.1 channel operation.
The extra one or two channels are for the back or rear surround channels. Both carry the same signal; the two back speakers the 7.1 amps drive simply spread the rear sound out more.
These 6.1 and 7.1 audio systems are at their best with programming that encodes a separate discrete rear sound signal (DTS-EX, etc.), but they will often improve a regular 5.1 audio source by synthesizing a back channel signal.
Your TV-watching seating arrangement will have a lot to do with your decision to go with five channels, or six or seven. Because the rear channel speakers have to be some distance behind you (ideally the same distance behind you as you main channel speakers are in front of you), they would be a non-starter if your sofa is backed up against a wall.
You can always buy a 6.1 or 7.1 channel receiver/amp and use it for five channels now, with the option of adding rear speakers later. Most better mass-market audio/video receivers are 7.1 these days; even many entry-level receivers are now 6.1 channels.
If you're thinking about buying a new amp/receiver now, you may want to wait a bit. The next generation audio standards from DTS and Dolby Labs have been announced and will be incorporated into the next generation high-definition HD-DVD and Blu-ray Disc players.
"Dolby TrueHD is Dolby's next-generation lossless technology developed for high-definition disc-based media. Dolby TrueHD delivers tantalizing sound that is bit-for-bit identical to the studio master, unlocking the true high-definition entertainment experience on next-generation discs. . . Supports up to eight full-range channels of 24-bit/96 kHz audio . . . Up to 18 Mbps bit rate." (from Dolby's web site)
DTS-HD (sometimes with "Master Audio" suffix) offers 7.1 discrete channels of 24-bit/192 kHz sound. ". . . indistinguishable from the original master" (from DTS promo material)
Note: Dolby Digital Plus is already available; it can carry multichannel audio programs of up to 7.1 channels and supports data rates as high as 6 Mbps.
These new standards will be compatible with existing receivers, but the sound won't be as good as with a receiver that has DTS-HD or Dolby TrueHD decoding built-in. Expect to see these more capable standards in the high-end receivers first, and then trickle down to everything else. The new HDMI 1.3 interconnect technology is required, and therefore this technology will not show up in A/V receivers until 2007.
Subwoofers. Most modern full-range speakers do not put out frequencies much below 50 Hz, if that. If you intend to watch big action or sci-fi movies, listening to the audio without a subwoofer will be like listening to stereo with one channel turned off. The difference is like night and day, especially if you have a gutsy subwoofer that will kick out sound down to 20 Hz before output starts to drop off.
If you watch only talk shows and the news, on the other hand, or live in an apartment with thin walls, then buying a big subwoofer may not be a good idea. Maybe just a small one...